Color printing process and product

ABSTRACT

A seven color separation process is provided in which, as well as the conventional Cyan, Magenta, Yellow and Black (CMYK) separations traditionally used in the four color printing process, additional Red, Green and Blue (RGB) separations are produced on a conventional scanner. The additional separations are produced using the Cyan, Magenta and Yellow channels of the scanner to measure Red, Green and Blue absorption characteristics of the source and to compensate the measured densities such that the separations represent a range of print densities from 0.0 D to 2.0 D corresponding to densities in the source in the range 1.0 D to 3.0 D. The transfer duration is also compensated for the compression which takes place in the preparation of the CMYK separations.

INTRODUCTION

This invention relates generally to the art of colour printing, and inparticular, to a process of automatic colour separation whereby anextended print density range may be achieved.

BACKGROUND OF THE INVENTION

Current methods of colour separation for colour printing involve the useof four colours to reproduce an original colour print or transparency.The standard four colours used are cyan, magenta, yellow and black(CMYK).

Such methods of printing have been in operation since the early part ofthe twentieth century and have permitted the reproduction on paper of anoriginal colour image, although with limited colour range and saturationfor matching colours in the reproduction to the original.

Unfortunately, saturated colours such as deep reds, greens and bluescannot be reproduced satisfactorily due to the limited print range offour colour process.

In recent years, there has been a strong demand for up to seven colourseparations, in order to reproduce as closely as possible the colours inthe original. This is a time-consuming task in which originals arescanned in the traditional CMYK method on a colour separation scanner.

Electronic colour data generated is then sent to a colour retouchingsystem and colour masked to "create" additional special colours, ie.red, green and blue, to produce seven colour separations.

This process relies heavily upon the skill of the operator and the powerof the colour retouching system to interpret the amount of colourrequired in each particular area of the new colour separations, to matchclosely to the colours in the original. It is not uncommon to makeseveral attempts at matching the colours in the original during thecourse of this process, which involves intensive use of labour andmaterials.

SUMMARY OF THE INVENTION

According to a first aspect, the present invention provides a method ofproducing colour separations for a printing process wherein an imagesource is scanned one or more times to produce a plurality of datachannels each of which provides a representation of one colourseparation of the image source, the data provided by each channel beingrestricted to represent a printable tone density range, and the channelsbeing separated into two groups, a first providing separation datarepresenting a base image and comprising a compressed representation ofthe density range of the image source, and the second providingseparation data representing a saturation image and substantiallycomprising a representation of the density range data lost from thefirst data set as a result of tone compression to a printable densityrange.

According to a second aspect the present invention provides a method ofproducing colour separations for a printing process comprising the stepsof:

(a) scanning an image source with a scanner having a conventional set ofscanning parameters to produce data representing a first set of colourseparations of the image, each having a printable density range, theseparations being capable of reproducing the image with a tone densityrange within a printable density range;

(b) reprogramming the scanner to alter the scanning parameters toidentify saturated colours in the image source;

(c) scanning the image source to produce a second set of datarepresenting a second set of colour separations of the image also havinga printable density range and carrying saturation colour information notrecorded in the first set of data; and

(d) preparing a set of colour separations from the first and second setsof data.

According to a third aspect the present invention provides a colourseparation scanner comprising an image source mounting means,illumination means arranged to illuminate an image source mounted on themounting means, optical separation means arranged to separate lightcomponents reflected from or transmitted through the image source into aplurality of colour components, detection means arranged to detect eachof the plurality of colour components and to convert each component intoan electrical colour signal, and signal processing means arranged toconvert each colour signal into a compressed colour signal representinga colour separation having a printable tonal range wherein theprocessing means is arranged to produce two types of colour signals afirst of which represents a print density range of 0-100% and iscompressed by a first compression function from source image tonaldensities in a range of 0.0 to 3.0 D and the second of which representsa print density range of 0-100% and is produced by applying a secondfunction to source image tonal densities in an upper portion of the 0.0to 3.0 D range, the second function, which produces the second type ofcolour signal, being complementary to that of the first compressionfunction to reproduce tonal detail lost by the first compressionfunction.

In the preferred embodiment the image source is scanned two or moretimes but other embodiments are possible in which the first and seconddata sets are produced simultaneously using one scan of the imagesource.

The colour separations prepared from the first set of data will beconventional cyan, magenta, yellow and black (CMYK) separations.

The separations prepared from the second set of data will typically bered, green and blue (RGB) although this would be dependent upon thesaturated colours present in the image source and in some circumstancesone or more of the red, green and blue separations may be replaced by asuitable selection of magenta, yellow and cyan separations respectively.

Preferably, a density function f"'(d) of each data channel of the secondset is given by f"'(d)=f'(d)-f"(d) where f'(d) is the desired finaldensity function and f"'(d) is the density function of the correspondingseparation or combination of separation from the first set.

Preferably, red or magenta separation data of the second set isprocessed by measuring green and blue absorbtion using the cyan channelof the scanner, the green or yellow separation data of the second set isprocessed by measuring red and blue absorbtion using the magenta channelof the scanner, and the blue or cyan separation data of the second setis processed by measuring red and green absorbtion using the yellowchannel of the scanner.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the invention will now be described, by way of example,with reference to the accompanying drawings in which:

FIG. 1 graphically illustrates a density transfer function applicable tothe preparation of separations having a 0.0 to 2.0 D tonal density rangefrom a reflective source image having a 0.0 to 2.0 D tonal range.

FIG. 2 graphically illustrates a compression function used in thepreparation of four colour CMYK process separations having a 0.0 to 2.0D tonal density range from a transparency having a 0.0 to 3.0 D tonaldensity range.

FIG. 3 graphically illustrates:

I--a unity transfer function having an output (print) tonal densityrange of 0.0 to 3.0 D;

II--The compression function employed in a traditional four colourseparation process; and

III--A transfer function for neutral density regions with 80% undercolour removal in the saturated colour areas.

FIG. 4 schematically illustrates a dichroic assembly of a four colourseparation scanner.

FIG. 5 diagramatically illustrates the relationship between variouscolours of the spectrum.

FIG. 6 diagrammatically illustrates the filtration and compressionprocess when scanning cyan, magenta, yellow and neutral densityseparations in a conventional CMYK scanner;

FIG. 7 diagrammatically illustrates the filtration and compensationprocess when scanning red, blue and green saturation separations in aprocess according to the present invention.

FIG. 8 is a table showing colour correction values applicable whenproducing red, green and blue saturation colour separations on aCrosfield 6×6 series Magnascan scanner.

FIG. 9 is a table showing colour correction values applicable whenproducing magenta, yellow and cyan saturation colour separtaions onCrosfield 6×6 series Magnascan scanner.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

In one form of the invention, the process of the invention makes use ofa second scan of the source material after the first scan is made in thetraditional four colour process, the scanner channels being reprogrammedto detect saturation colour information, however, it is also possible toperform the second scan concurrently with the first if the scanner werespecifically redesigned.

Typically, embodiments of the inventive process will produce specialred, green and blue colour separations, from colour transparencies orreflective originals, on a conventional colour separations scanner, tocompliment the existing four colour system traditionally used in thegraphic arts industry. However, in some instances it will be preferableto produce additional cyan, magenta or yellow separations depending uponthe predominant colours in the image source.

Four Colour Process only allows the reproduction of a limited densityrange between 0.0 an 2.0 D. In printing terms this equates to between 0%and 100% maximum ink on paper utilising conventional halftone screeningtechnology.

Reflective scource material such as photographic prints typically havean average dynamic range of 2.0 D and can therefore be reproducedreadily in four colour process without loss of tonal detail using aunity transfer function as illustrated in FIG. 1.

Colour transparencies however, typically have an average dynamic rangeof 3.0 D. Therefore the dynamic range of a transparency is greater thanthe print range of 2.0 D. Compensation must be made for this differenceby way of tone compression to reproduce the original transparency withinthe limitations of the print range. The necessary tonal compressionfunction to achieve this is illustrated in FIG. 2. The printed result ineffecting this tone compression will show loss of detail and saturationin the shadow and saturated colour regions of the original.

The saturated hues of red green and blue fall into this area and aremost noticeable to the observer of the reproduction to the original.

The brighter hues of cyan, magenta and yellow do not typically createthe same problem to reproduce as their transmission densities fallwithin the printable tone range of 0.0 to 2.0 D. On the other hand, thehues of red, green and blue can achieve densities greater than 2.0 D,hence the need to print additional red, green and blue ink to capturethe saturation densities of these colours in the final printedreproduction.

In theory, using the traditional four colour process, you would need toprint up to 150% ink in each colour separation to reproduce a dynamicrange of 3.0 D.

In practise this cannot be achieved due to current halftone screentechnology and printing inks which only allow an average print range of2.0 D or up to 100% ink printing in each colour separation.

The addition of special red, green and blue colour separations willsolve this problem by recording these colours in the density rangebetween 1.0 and 3.0 D where normal CMYK tone compression has beenapplied.

This is achieved in the following method:

Step 1. Scan the original in the traditional four colour methodeffecting 50% polychromatic colour removal.

Step 2. A second scan must be made from the same source original torecord the density range between 1.0 and 3.0 D. In practice, thefunction applied to the data produced by the second scan isf"'(d)=f'(d)-f"(d) where f'(d) is the desired tonal range of the finalprint image and f"(d)is the compression function of the first scan. (NB:From this point on only the cyan, magenta and yellow channels on thescanner are required--the black channel is not used).

In the following description, the additional cyan, magenta and yellowseparations will print red, green and blue ink respectively in the finalreproduction.

The second scan is achieved by effecting the first printing tones ofcyan, magenta and yellow to 0% at a preferred tonal density of 1.0 Dwhile setting the last printing tones of 100% cyan, magenta and yellowat a preferred tonal density of 3.0 D. The print range of 2.0 D has nowbeen moved up the tonal scale to record the input densities between 1.0and 3.0 D.

Under Colour Removal

Under colour removal removes tonal density from the separation in areaswhere the source image has a neutral density component. For thesaturation (second) scan apply 80% under colour removal with a startingpoint of 0% at 1.0 D. This will remove the unwanted neutral componentfrom the original leaving approximately 20% colour in the neutral areasof each separation to increase saturation and definition.

Colour Correction

There are 18 colour areas which must be corrected to achieve the desiredeffect in the final printed result.

Using the colour correction function remove all unwanted colours fromthe reproduction by aligning the tonal densities in each colour area andadjusting the cyan, magenta and yellow ink percentages as shown in FIG.8

After making the colour adjustments the scanner will be calibrated torecord the ink percentage required to reproduce the red, green and bluesaturation components in the source original using the cyan, magenta andyellow channels respectively.

As a standard it is preferred to print the special cyan(red)magenta(green) and yellow(blue) separations in the following Pantonecolours:

CYAN=WARM RED

MAGENTA=GREEN PMS 354

YELLOW=BLUE PMS 293

An alternative colour correction arrangement is shown in FIG. 9 for acase where the second set of separtions is prepared for printingMagenta, Yellow and Cyan instead of Red, Green and Blue respectively.This is desirable when the original has a predominance of one or it isnot necessary to substitute all three colours, and for example only Cyanmight be substituted for Blue when the original has a predominant brightBlue sky.

Grey Balance

All grey balance is removed and the yellow and magenta separations areadjusted to be identical in dot percentage to the cyan separation.

Unsharp Masking (USM)

The unsharp masking function should be suppressed.

The result of effecting this inventive process will dramaticallyincrease colour saturation and contrast in the final printed product.

The scanner can be programmed to carry out these steps using colourcorrection tables as reference. The programming of the tables can bemodified as required.

To assist in understanding the working of the invention, the operationof dichroic filters in a dichroic assembly of a standard four colourseparator will be described with reference to FIG. 4 of the drawings.

In a conventional colour separation scanner, the image source isilluminated from above or behind depending upon whether a reflectionsource or transparency is being scanned. A scanning head is thensystematically moved over the image source to measure in turn the light11 emitting from each point on the surface of the image source.

The beam of the light 11 emitted from the image source is then reflectedthrough a series of mirrors until it reaches the dichroic assembly ofFIG. 4. Entry into the dichroic assembly is via a mirrored prism 12which directs the beam 11 through balancing filters 13 and onto a seriesof coated dichroic mirrors 14,15, which selectively reflect one colourand pass the others. In this manner, a red light component 17 isreflected by a first dichroic mirror 14, a blue light component 18 isreflected by a second dichroic mirror 15 and a green light reflected bya second dichroic mirror 15 and a green light component 19 is reflectedby a final conventional mirror 16. The red, blue and green lightcomponents 17,18,19 are then filtered through red green and blueseparation filters 21,22,23 before striking photomultipliers for theCyan channel 24, the Yellow channel 25 and the Magenta channel 26.

The photomultipliers measure the intensity of light striking them andconvert this into an electrical signal which is then digitized andprocessed by a programme running on a microprocessor.

The Cyan photomultiplier 24 measures the intensity of red light emittedfrom the source image and the inverse of this signal is therefore ameasure of the red light absorbed by Cyan (Blue and Green) pigment inthe source. Similarly, the Yellow multiplier 25 measures the intensityof Blue light reflected or Yellow (Red and Green) light absorbed and theMagenta photomultiplier 16 measures the intensity of Green lightreflected or Magenta (Red and Blue) light absorbed. The relationshipbetween reflected/absorbed colours in the spectrum is illustrated inFIG. 5.

FIG. 6 diagramatically illustrates the light and signal processingperformed in a conventional separation scanner. In this diagram,polychromatic light is reflected from or transmitted through the sourceand processed by the dichroic assembly to obtain red blue and greenlight components.

The red blue and green components are then measured by cyan magenta andyellow channel photomultipliers to produce cyan, magenta, yellow andblack signals. It should be noted that these signals actually representthe data required to produce cyan, magenta, yellow and black negativesand therefore signal inversion does not actually take place at thispoint. A compression function f"(d) is then applied to each channel toreduce the dynamic range of each signal to fall within the printablerange of 2 D, and these compressed signals are then used as exposuresignals to produce four separation negatives, for Cyan, Magenta, Yellowand Black print separations. It will be noted that black is printed whenred, green and blue are all absent (ie cyan, magenta and yellow are allpresent) and density is electronically removed from the cyan, magentaand yellow separations in proportion to the density added to the blackseparation. The proportion of density moved to the black separation iscontrollable by the operator of the scanner and is called under colourremoval.

Turning to FIG. 7, a similar diagram to that of FIG. 6 is provided toshow the processing steps required in a conventional scanner to producethree saturation separations for printing red, green and blue ink.

As with FIG. 6, the dichroic assembly separates red, green and bluelight components emitted from the source and these are measured by thecyan, magenta and yellow channel photomultipliers. The output signalsfrom the photomultipliers are then processed to determine the red, greenand blue saturation components as follows:

Red component--coupled detection of the blue and green (cyan) absorptionthrough the cyan channel;

Blue component--coupled detection of the red and green (yellow)absorption through the yellow channel;

Green component --coupled detection of the red and blue (magenta)absorption through the magenta channel;

These signals are then compensated to counteract the compressionfunction applied to the first (CMYK) scan. By measuring absorption ofthe complementary colour rather than transmission of the colour ofinterest, an inversion is automatically included in this processing, asindicated by the -f'(d) component, to cancel the inversion which takesplace in the production of the separation negative.

To aid in illustrating a preferred embodiment of the invention, thefollowing tables set out an example of technical data required toproduce special red, green and blue colour separations on any CrosfieldMagnascan 6×6 series scanner.

Table 1

CAL 130 Recall full block "0"

Table 2

Density Range 2.0 D

CAL 28 1.0 Yellow

1.0 Magenta

1.0 Cyan

Manual enter white cyan 0%

CAL 29 3.0 Yellow

3.0 Magenta

3.0 Cyan

Manual enter black cyan 100%

Table 3

Grey Balance

CAL 13 cyan=magenta=yellow

STEPS:

2/5%, 3/10%, 4/15%, 5/20%, 6/25%, 7/30%, 8/40% 9/50%, 10/60%, 11/70%,12/80%, 13/85%, 14/90%, 15/95%

Table 4

Colour Correction

CAL 12

STEPS:

1/-2.00, 2/-2.00, 3/-2.00, 4/0.00, 5/+1.20, 6/0.00, 7/-2.00, 8/0.00,9/+1.20, 10/0.00, 11/-2.00, 12/-1.00, 13/+1.20, 14/-1.00, 15/-2.00,16/-1.00, 17/-2.00, 18/0.00,

Steps 19 through 24 set to 0.0.

Table 5

Under Colour Removal

CAL 515

Breakpoint--0

Slope--0.20

Table 6

Unsharp Masking (USM)

Sharp--0

Smooth--0

Table 7

Black Printer

CAL 11

Offset--+2.0

Slope--0.0

ALL OTHER DATA REMAINS UNCHANGED IN THE VERSION 9.0 SOFTWARE

It will be apparent that changes in, and modifications to the inventionmay be made without departing from the spirit and scope thereof.

I claim:
 1. A method of producing colour separations for a print processwherein an image source is scanned one or more times to produce aplurality of data channels each of which provides a representation ofone colour separation of the image source, the data provided by eachchannel being restricted to represent a printable tone density range,and the channels being separated into two groups, a first providingseparation data representing a base image and comprising a compressedrepresentation of the density range of the image source, and the secondproviding separation data representing a saturation image andsubstantially comprising a representation of the density range data lostfrom the first data set as a result of compression to a printabledensity range.
 2. The method of claim 1 wherein the image source isscanned once and the plurality of data channels are simultaneouslyproduced.
 3. The method of claim 1 wherein the image source is scannedtwo or more times.
 4. The method of claim 1 wherein the first data setrepresents cyan, magenta, yellow and black colour separations.
 5. Themethod of claim 1 wherein the second data set represents red, green andblue colour separations.
 6. The method of claim 1 wherein the seconddata set represents one or more colour separations selected from red,green, blue, cyan, magenta and yellow colour separations.
 7. The methodof claim 1 wherein a density function f"'(d) of each data channel of thesecond set is given by f"'(d)=f'(d)-f"(d) where f'(d)is the desiredfinal density function and f"(d) is the density function of thecorresponding separation or combination of separations from the firstset.
 8. The method of claim 1 wherein red or magenta separation data ofthe second set is processed by measuring green and blue absorption usingthe cyan channel of the scanner, the green or yellow separation data ofthe second set is processd by measuring red and blue absorption usingthe magenta channel of the scanner, and the blue or cyan separation dataof the second set is processed by measuring red and green absorptionusing the yellow channel of the scanner.
 9. A method of producing colourseparations for a print process comprising the steps of:(a) scanning animage source with a scanner having a conventional set of scanningparameters to produce data representing a first set of colourseparations of the image, each having a printable density range, theseparations being capable of reproducing the image with a tone densityrange within a printable density range; (b) reprogramming the scanner toalter the scanning parameters to identify saturated colours in the imagesource; (c) scanning the image source to produce a second set of datarepresenting a second set of colour separations of the image also havinga printable density range and carrying saturation colour information notrecorded in the first set of data; and (d) preparing a set of colourseparations from the first and second sets of data.
 10. The method ofclaim 9 wherein the first data set represents cyan, magenta, yellow andblack colour separations.
 11. The method of claim 9 wherein the seconddata set represents red, green and blue colour separations.
 12. Themethod of claim 9 wherein the second data set represents one or morecolour separations selected from red, green, blue, cyan, magenta andyellow colour separations.
 13. The method of claim 9 wherein a densityfunction f"'(d)of each data channel of the second set is given byf"'(d)=f'(d)-f"(d) where f'(d) is the desired final density function andf"(d) is the density function of the corresponding separation orcombination of separations from the first set.
 14. The method of claim 9wherein red or magenta separation data of the second set is processed bymeasuring green and blue absorption using the cyan channel of thescanner, the green or yellow separation data of the second set isprocessd by measuring red and blue absorption using the magenta channelof the scanner, and the blue or cyan separation data of the second setis processed by measuring red and green absorption using the yellowchannel of the scanner.
 15. A colour separation scanner comprising anoptical separation means arranged to separate light components emittedfrom an image, detection means arranged to detect each of the pluralityof colour components and to convert each component into an electricalcolour signal, and signal processing means arranged to convert eachcolour signal into a compressed colour signal representing a colourseparation having a printable tonal range wherein the processing meansis arranged to produce two types of colour signals a first of whichrepresents a print density range of 0-100% and is compressed by a firstcompression function from source image tonal densities in the range 0.0to 3.0 D and the second of which represents a print density range of0-100% and is produced by applying a second function to source imagetonal densities in an upper portion of the 0.0 to 3.0 D range, thesecond function, which produces the second type of colour signal beingcomplementary to that of the first compression function to substantiallyreproduce tonal detail lost by the first compression function.
 16. Thecolour separation scanner of claim 15 wherein the first and secondsignal types are generated in a single scan of the source image.
 17. Thecolour separation scanner of claim 15 wherein the first and secondsignal types are generated in separate scans of the source image. 18.The colour separation scanner of claim 15 wherein the signals of thefirst signal type represents cyan, magenta, yellow and black colourseparations.
 19. The colour separation scanner of claim 15 wherein thesignals of the second signal type represent red, green and blue colourseparations.
 20. The colour separation scanner of claim 15 wherein thesignals of the second signal type represent one or more colourseparations selected from red, green, blue, cyan, magenta and yellowcolour separations.
 21. The colour separation scanner of claim 15wherein the tonal density compression function f"'(d) of each coloursignal of the second type is given by f"'(d)=f(d)-f"(d) where d is thedensity of the source image, f(d) is the desired tonal density functionof a final printed image for a colour and f"(d) is the tonal densitycompression function of signals of the first type for the colourseparation or combination of colour separations producing the respectivecolour in the final printed image.
 22. The colour separation scanner ofclaim 15 wherein red or magenta separation data of the second set isprocessed by measuring green and blue absorption using the cyan channelof the scanner, the green or yellow separation data of the second set isprocessed by measuring red and blue absorption using the magenta channelof the scanner, and the blue or cyan separation data of the second setis processed by measuring red and green absorption using the yellowchannel of the scanner.
 23. A method of producing print by means whichutilizes colour separations produced by a method wherein an image sourceis scanned one or more times to produce a plurality of data channelseach of which provides a representation of one colour separation of theimage source, the data provided by each channel being restricted torepresent a printable tone density range, and the channels beingseparated into two groups, a first providing separation datarepresenting a base image and comprising a compressed representation ofthe density range of the image source, and the second providingseparation data representing a saturation image and substantiallycomprising a representation of the density range data lost from thefirst data set as a result of compression to a printable density range.24. Print produced by a method which utilizes colour separationsproduced by a method wherein an image source is scanned one or moretimes to produce a plurality of data channels each of which provides arepresentation of one colour separation of the image source, the dataprovided by each channel being restricted to represent a printable tonedensity range, and the channels being separated into two groups, a firstproviding separation data representing a base image and comprising acompressed representation of the density range of the image source, andthe second providing separation data representing a saturation image andsubstantially comprising a representation of the density range data lostfrom the first data set as a result of compression to a printabledensity range.